The present invention relates to a cooling unit for facilitating the radiation of heat from heat-generating components such as semiconductor packages, and also an electronic apparatus incorporating the cooling unit.
In recent years, portable electronic apparatuses, typified by book-like portable computers and mobile information apparatuses, have been developed. Each of these apparatuses incorporates a high-speed MPU (Microprocessing Unit) and has various functions in order to process multimedia information, such as characters, speech, sound, and images. The higher the integration density the MPU has and the more functions does the MPU perform, the more electric power it consumes while operating. The more electric power the MPU consumes, the more heat it generates. In the case of a portable computer incorporating an MPU, the heat generated by the MPU must be radiated efficiently from the MPU in the housing of the portable computer. It is therefore absolutely necessary to provide a cooling unit in the computer housing to cool the MPU.
A conventional cooling unit is known, which comprises a heat sink and an electric fan device. The heat sink is designed to receive the heat generated by an MPU and made of metal having high thermal conductivity, such as aluminum alloy. The heat sink has a base mounted on the top of the MPU. The electric fan device is secured to the upper surface of the base.
The electric fan device comprises a flat fan casing and a fan supported in the fan casing. The fan casing comprises a bottom wall, a side wall and a top wall. The bottom wall is mounted on the base. The top wall has an air inlet port. The side wall has an air outlet port, which communicate with the inlet port. The fan casing is positioned horizontally on the base, with the axis of the fan extending in the height direction of the computer housing. The cooling unit can therefore be thinner than otherwise.
When the fan of the fan unit is driven, cooling air is drawn toward the bottom wall of the fan casing, through the air inlet port. The cooling air flows along the bottom wall and is exhausted from the outlet port. Thus, the cooling air facilitates the radiation of heat from the MPU.
The cooling unit is, however, thick as a whole. This is inevitably because the heat sink is mounted on the top of the MPU and the fan device is mounted on the upper surface of the heat sink. In this connection it should be noted that a prominent commercial value of portable computers resides in their portability. The housing of a portable computer must therefore be as thin as possible. In view of this it is undesirable to provide a space above the MPU, in order to accommodate the cooling unit.
To make matters worse, a thermal resistance develops at the interface between the base of the heat sink and the bottom wall of the fan casing. This is because the heat generated by the MPU is transmitted from the base to the bottom wall. The thermal resistance renders it difficult to convey the heat to the fan casing with high efficiency. This decreases the efficiency of cooling the MPU.
Another type of a cooling unit that may help to reduce the thickness of computer housing is known. This cooling unit is characterized in that the heat sink and the fan device are arranged side by side. The cooling unit has a heat pipe, which thermally connects the heat sink and the fan casing.
In this cooling unit, the heat pipe transmits the heat generated by the MPU, from the heat sink to the fan casing. The stream of cooling air, generated as the fan device is driven, takes the heat out of the computer housing. Therefore, the heat generated by the MPU can be radiated from the computer housing, without need of mounting the fan device on the heat sink. This is why the cooling unit of this type helps reduce the thickness of the computer housing.
However, a thermal resistance may develop in this cooling unit, too. As mentioned above, the heat pipe thermally connects the heat sink and the fan casing. Depending on how the heat pipe is coupled to the heat sink and to the fan device, a thermal resistance may exists at the node between the heat pipe and the heat sink or at the node between the heat pipe and the fan device, or at both nodes. The heat resistance, if any, may impair the heat transmission from the MPU to the heat pipe and from the heat pipe to the fan casing. Hence, the cooling unit needs to be improved furthermore.